Preheat oven to 375°F (190°C).

Season the chicken breasts with salt and black pepper.

In a skillet, melt butter over medium heat. Add mushrooms and cook for 5 minutes until softened.

Add garlic and thyme to the mushrooms, cook for an additional 2 minutes.

Stir in spinach and cook until wilted. Remove from heat and let cool slightly.

Cut a pocket in each chicken breast and fill with the mushroom mixture and cheese. Secure with toothpicks if needed.

Heat olive oil in a skillet over medium-high heat. Sear chicken breasts until golden brown on each side.

Transfer the skillet to the preheated oven and bake for 15 minutes or until chicken is cooked through.